Altitude can be measured purely with the aid of atmospheric pressure, because the air pressure of the atmosphere changes as a function of altitude. This means that variations in atmospheric pressure that are not caused by a change in the elevation of the sensor will nevertheless introduce a significant error source to the measurement. In such devices, the base level of the atmospheric pressure should also be manually calibrated, in order to define absolute altitude. In addition to atmospheric pressure, variations in temperature can also decisively affect known measurement methods based on atmospheric pressure.
As an example of a drawback in known methods, it is possible to refer to a situation, in which a rapid climb takes place simultaneously with a rapid temperature change. In that case, a large error can be included in the altitude reading after the climb, even though the reading might have been entirely correctly calibrated before the climb, and major changes did not take place in the general weather conditions in the area. Should the weather conditions also change at the same time; the error can be even greater.
To allow for error sources, various methods have been developed in determining altitude, in which a second sensor is utilized in addition to a barometric sensor. For example, with the aid of an acceleration sensor it is possible to detect whether a person is stationary or moving, and, on the basis of this information, the information provided by the barometric sensor can be filtered. On the other hand, FI Publication No. 119297 discloses a method, in which the Earth's magnetic field being also measured using a magnetic sensor, pressure information is measured, and the pressure information is used both as atmospheric-pressure information and to determine the elevation. Consecutive measurements are performed using the magnetic sensor, and the consecutive measurement results are compared and, on the basis of the comparison, the mobile device is directed to either atmospheric-pressure measurement or altitude measurement, or the desired altitude is set for the mobile device.
U.S. Pat. No. 7,324,002 discloses the giving of a weather alert based on determining elevation, with the aid of a wrist-top device.
In several known methods a satellite-positioning sensor is utilized. U.S. Pat. No. 6,055,477 discloses the calculation of the offset error of an altimeter (e.g., based on atmospheric pressure) with the aid of GPS altitude measurement. The offset error of the altimeter is calculated with the aid of a single-state Kalman filter.
For example, U.S. Pat. No. 6,522,298 discloses a method for calibrating an altimeter with the aid of GPS (Global Positioning System). In one embodiment of the method, changes in pressure are measured and a mean value is calculated recursively for an error estimate of the pressure at zero altitude (reference altitude). If changes are detected with the aid of the error model, which cannot be explained by changes in altitude, it is concluded that they have been caused by the environment.
U.S. Pat. No. 6,768,449 discloses a method for calculating a calibrated altitude, which method utilizes the pressure at a reference altitude. In one embodiment, an error model of the pressure at a reference altitude is updated with the aid of difference between GPS and pressure measurements.
In U.S. Pat. No. 7,142,152, a calibration model of the pressure at a reference altitude is again used, which takes into account not only the static offset error, but also the dynamic error, i.e. the error change in the time plane.
A common factor in the methods described in U.S. Pat. Nos. 6,522,298, 6,768,449, and 7,142,152 is that the barometric altitude is always calculated. On the basis of the barometric altitude and the GPS altitude, the calibrated pressure at sea-level is calculated (the reference level of the pressure). The final altitude based on pressure is then calculated with the aid of the calibrated sea-level pressure.
It can be said generally that many known methods rely heavily on a GPS-calibrated, but nevertheless strong atmospheric-pressure-based definition, and in many situations their ability to allow for error sources relating especially to temperature and changes in weather is poor. There is therefore a need for new, improved methods.